1. Technical Field of the Invention
The present invention relates to a tire for a vehicle wheel in which at least one of the beads comprises a seat having a generatrix, the axially inner end of which lies on a circle of a diameter greater than the diameter of the circle on which the axially outer end is located. This type of design is particularly suited to the new generations of tires which can be used, within certain limits, in conditions of low pressure, or even zero or virtually zero pressure, with the risk of separation of the tire from the rim on which it is mounted being eliminated. This concept is frequently designated by the expression xe2x80x9cextended mobilityxe2x80x9d.
2. The Related Art
For a long time, tire manufacturers have been trying to develop a tire which does not create any source of potential risk or danger in the event of an abnormal drop in, or even total loss of, inflation pressure. One of the difficulties encountered relates to travelling with a flat tire or at very low pressure. In fact, when travelling at very low pressure, or even at zero pressure, with conventional tires, the beads are at great risk of separating from the periphery of the rim against which they are held by the inflation pressure.
Numerous solutions have been tested in order to overcome these disadvantages. Frequently, these solutions cause additional difficulties in mounting/demounting the tire on/from the rim.
EP 0 582 196 (U.S. Pat. No. 5,660,656) discloses a tire comprising a tread extended by two sidewalls and two beads and also a carcass anchored in the two beads to an annular reinforcement. The carcass is formed of cords in an adjacent arrangement, which are aligned circumferentially and are in contact with at least one layer of bonding rubber of very high elasticity modulus in the hooking zone of the bead comprising the annular reinforcement. In this tire, the annular reinforcement of the hooking zone of the bead is formed of stacks of circumferential cords with the interposition of a layer of bonding rubber of very high elasticity modulus between the reinforcement cords of the carcass and the stacks of circumferential cords.
This embodiment is intended for tires of conventional type, with the beads being held against the rim hook due to the inflation pressure of the tire. In this type of arrangement, there is a predominance of stresses of the lateral or axial type, which induces major compressive forces that act substantially axially from the sidewalls towards the center of the bead. These forces increase according to the inflation pressure. The increase in pressure tends to make the bead slide against the hook, radially towards the outside. The stresses induced radially towards the inside, against the seat of the rim, decrease with an increase in pressure, or with any increase in the tension of the carcass-type reinforcement structure.
It will furthermore be noted that the stacks of cords are aligned in a direction substantially parallel to the orientation of the profile of the rim hook against which the bead bears.
The profile of the bead of this type of tire is relatively narrow and elongated; the anchoring is distributed over the major part of the height and width of the bead. The passage of the carcass into the bead is generally substantially central relative to the walls of the bead.
Furthermore, when a relatively narrow bead is subjected to predominantly axial stresses, neither the inflation pressure nor the tension induced in the carcass permits the generation of large moments or torques, which tend to make the bead pivot or turn on itself.
With such a type of tire, if the pressure drops and the vehicle continues to travel, the holding of the tire on the rim is no longer ensured, and in the majority of cases it rolls off the rim.
EP 0 673 324 (U.S. Pat. No. 5,634,993) describes a rolling assembly comprising at least one tire with a radial carcass reinforcement which is anchored within each bead and a rim of specific shaping. This rim comprises a first seat, with a generatrix such that the axially outer end of the generatrix is spaced from the axis of rotation by a length less than the spacing between its axially inner end and the axis of rotation, and is defined axially to the outside by a protrusion or rim flange. The tire comprises bead seats suitable for mounting on the rim. The type of tire/rim interface proposed in this document has many advantages compared with the solutions already known, in particular with regard to the ease of mounting/demounting, while making it possible to travel a certain distance despite a drop in pressure.
The present invention is directed in particular at proposing certain types of architecture in order to optimize the qualities of the assembly proposed in the document referred to above.
EP 0 748 287 (U.S. Pat. No. 6,179,028) describes a solution which permits initial optimization of the basic technology described in EP 0 673 324 referred to above. This is a tire, at least one bead of which has a structure that makes it possible to modify the clamping of the bead according to the tension of the carcass reinforcement and, in particular, reinforcement thereof when the inflation pressure increases to its rated value. The document thus proposes using a bead with anchoring of the end of the carcass by turning it up about the base of the bead wire, via the axially and radially inner sides relative to the bead wire. The bead also comprises, adjacent to the bead wire and axially to the outside thereof, a profiled element of rubber mix of relatively high hardness against which the bead wire can exert a compressive force when the tension of the carcass reinforcement increases. This compressive force creates self-clamping of the toe of the bead on the mounting rim. The tension of the carcass therefore involves displacement of the bead wire towards the outside, so that the latter generates the compressive force. In such a configuration, the presence of a bead wire of conventional type and the turning-up of the carcass beneath the latter are presented as being indispensable for generating the compressive force. This restricts the other types of arrangement which can be considered
Moreover, EP 0 922 592 describes two embodiments with the carcass anchored by turning it up axially towards the outside. The first embodiment proposes anchoring of the carcass in the bead by turning it up radially towards the outside of the end of the carcass. The upturn is surrounded on either side by two radially superposed layers of metal wires arranged axially side by side and covering substantially all of the axial portion along the seat of the bead. The layers are arranged so as to be parallel to the seat. The types of cords and the corresponding dimensions are very precise.
The second solution proposed in this document relates to bead seats with different diameters. The securing of the carcass is also effected differently from the first solution. First of all, the carcass is subdivided into two portions which are radially separated at the level of the bead. Each portion is adjoined by a layer of cords which is arranged radially, each layer being arranged radially to the outside against each of the carcass portions. The radially outer carcass portion and the layer of cords radially to the inside are separated by an insert of the type of elastomer of high hardness provided in the bead. This insert axially lines the central portion of the bead and rises radially towards the outside and axially towards the inside, beyond the radial limit of the presence of the metal wires.
The two examples of solutions of EP 0 922 592 have several disadvantages. Thus, the securing of the carcass proposed in this document requires the presence of an upturn axially towards the outside of the end portion of the carcass. Furthermore, the superposed layers of cords are arranged radially close to the seat of the bead, for a good part at a radial position closer to the axis of rotation than the upper portion of the flange on which the bead bears. Unless highly extensible cords are used, it is difficult to mount/demount the tire, due to the unfavourable radial position of the cords. It will also be noted that the stacks are oriented substantially parallel to the profile of the seat against which the bead bears.
According to the second solution, the carcass is subdivided into two portions and an insert of high hardness is necessary to separate, on the one hand, the layers of cords and, on the other hand, the two carcass portions. However, the carcass is not anchored in the insert. The form of the insert described is limitative.
It is an object of the present invention to overcome the various disadvantages inherent in the solutions set forth above.
To do this, it provides a tire for a vehicle wheel, comprising:
two sidewalls spaced apart axially from each other, joined at their radially outer portions by a crown zone provided on its radially outer portion with a circumferential tread;
a bead, arranged radially to the inside of each of the sidewalls, each bead comprising a seat and an outer flange which are intended to come into contact with a suitable rim;
a reinforcement structure extending substantially radially from each of the beads, along the sidewalls, towards the crown zone;
at least one of the beads comprising:
a bead seat comprising a generatrix, the axially inner end of which lies on a circle of a diameter greater than the diameter of the circle on which the axially outer end is located;
an anchoring zone for the reinforcement structure in the bead, comprising a substantially radial arrangement of circumferential cords, the anchoring zone being arranged in the substantially radially inner portion of the bead;
an intermediate zone, arranged substantially axially externally to the anchoring zone and comprising at least one portion which is radially external relative to the radially innermost portion of the anchoring zone;
a bearing zone for the bead which is able to lie directly or indirectly against the suitable rim seat, and extending substantially axially along the bead seat and comprising at least one portion arranged substantially radially internally to the intermediate zone; and
the intermediate zone comprising a structure which permits the transfer of forces from the anchoring zone to the bearing zone of the bead so as to increase the bearing force of the seat of the bead in its axially outer portion when a substantially radially external tensile force is exerted on the reinforcement structure at least at the level of the bead.
Such a configuration permits optimum holding of the bead on the rim, in particular at the level of the rim seat/bead seat interface. The forces acting on the anchoring zone are then transmitted effectively to the zone of the seat of the bead. The radial continuity makes it possible to minimize the impact of the shearing forces acting against the bead in its entirety.
The anchoring zone does not comprise a bead wire, in particular a bead wire of conventional type, such as, for example, a multi-cord bead wire against which a carcass ply is turned up, this bearing or contact or cooperation effecting the holding of the carcass.
Each of the zones taken in isolation, and also all the zones together, to some extent forms an internal bead capable of effecting relative movements, such as, for example, of the angular or rotational type, relative to another zone, or relative to a virtual center of pressure CP, or relative to the seat of the rim, etc.
Advantageously, the intermediate zone ensures a mechanical connection between the anchoring zone and the bearing zone. The intermediate zone ensures continuity between the other two zones, such that the mechanical forces are transmitted from the anchoring zone towards the bearing zone.
The intermediate zone is able to exert a force substantially radially towards the inside against the axially outer portion of the bearing zone.
The arrangement of circumferential cords contributes to the circumferential clamping force of the tire against the rim on which it is mounted. The level of clamping is determined so as to ensure a good compromise between the ease of mounting/demounting and the assurance of reliable, durable mounting.
The intermediate zone ensures a mechanical connection between the anchoring zone and the bearing zone. The anchoring zone and the intermediate zone may be twinned mechanically. The same applies to the intermediate zone, which may be joined to the bearing zone. However, the anchoring zone is preferably not directly joined to the bearing zone.
The reinforcement structure of the sidewalls and the crown is advantageously of the carcass type, its mechanical properties being similar to those of carcass plies of known type. Furthermore, this reinforcement structure is advantageously formed without axial separation at the level of the bead. Thus all of the cords of the circumferential arrangement preferably occupy a substantially identical axial position.
Preferably, the anchoring zone is radially external to the bearing zone.
The transfer of forces advantageously takes place by a moment of force.
The anchoring zone of the reinforcement structure in the bead is advantageously formed at least in part of a rubber mix of substantially high modulus. This modulus may for example be substantially equal to or greater than 20 MPa, and preferably greater than 40 MPa. The presence of this type of rubber mix contributes to the anchoring of the carcass-type reinforcement structure. The reinforcement structure is at least partially in contact with this mix, or even preferably at least partially embedded in the mix.
Preferably, the bearing zone is substantially elongated. It is extended, for example, substantially along the seat of the bead. The transfer of forces upon rotation of the bottom zone of the axially inner portion of the bead towards the axially outer portion of the bead is thus possible, while maintaining bearing pressure against at least a portion of the seat of the bead. The transfer of forces creates self-clamping of the toe of the bead against the rim.
The bearing zone is preferably substantially adjacent to the rim seat.
Preferably, the anchoring zone is arranged in the portion of the bead which is substantially in the immediate vicinity of the carcass-type reinforcement structure. It may therefore be located either on only one or on both sides of the reinforcement structure.
Preferably, the bearing zone is substantially formed of a rubber mix of high modulus. The loading of this zone is thus limited and the contact with the seat of the rim is optimized. The modulus may for example be substantially equal to or greater than 20 MPa, and preferably greater than 40 MPa.
Preferably, the intermediate zone is substantially formed of a rubber mix of high modulus. It may advantageously be surrounded or encircled by one or more cords arranged around the zone, for example at 90xc2x0, so as to apply compressive stress to the zone. This modulus may for example be substantially equal to or greater than 20 MPa, and preferably greater than 40 MPa.
According to another advantageous variant, the bead also comprises a loadable buffer zone, which is arranged in the radially inner and axially outer portion and which is capable of bearing against a flange of the rim. The presence of such a buffer zone promotes the rotation of the bottom zone and also makes it possible to secure the axial hooking of the bead against the rim flange. The presence of a zone of rubber of high modulus in a radially inner portion relative to the flange provides good axial holding and prevents the bead from sliding axially towards the outside.
According to another advantageous variant, the bead portions generally around the anchoring, intermediate and bearing zones are generally occupied by a material having an elasticity modulus which is lower than that of the material of the anchoring, intermediate and bearing zones. These various zones, in fact, have a certain relative mobility relative to the rim seat on which the bead is mounted. This mobility is often expressed by displacements of the angular or rotational type. Each of the zones also has a certain relative mobility relative to the adjacent portions of the bead, which are formed of rubber mixes of lower modulus, and/or relative to the other neighbouring zones. This mobility can expressed, for example, by displacements of the angular or rotational type.
Preferably, the elasticity modulus of the rubber mix of high modulus is greater than 20 MPa, and preferably greater than 40 MPa.
According to another advantageous example, the bearing zone and/or the intermediate zone is/are arranged and defined such that upon substantially axially external rotation of the bottom zone of the tire, none of the zones exerts a force against the rim flange or hook which would be liable to cause disassembly of the bead of the tire. Thus, for example, the axially outer limit of the zones corresponds substantially to the outer limit of the rim on which the tire is likely to be mounted.
According to another advantageous variant, the intermediate zone comprises a substantially radial arrangement of circumferential cords. Similarly, the bearing zone may comprise a substantially radial arrangement of circumferential cords.
Advantageously, the number of axially inner cords of the anchoring zone is at most 1.5 times the number of axially outer cords. These are cords which are axially to the inside or the outside relative to the carcass-type reinforcement structure.
Preferably, the bead of the tire comprises a carcass-type reinforcement structure which extends substantially radially from each of the beads, along the sidewalls, towards the crown zone, each bead being reinforced by a substantially radial arrangement of circumferential cords arranged in stacks, a portion of the carcass-type reinforcement structure being arranged in the immediate proximity of at least one portion of a stack, and the space between such portions generally being filled by a rubber mix of high elasticity modulus.
An interface zone is advantageously formed by a portion of the reinforcement structure and at least one stack, such zone being generally embedded in a rubber mix of high modulus. Advantageously, the interface zone generally covers the stacks and the adjacent portion of reinforcement structure. Such an arrangement, in particular at the level of the bead, permits very great flexibility in defining the various zones comprising mixtures of different natures and/or characteristics. The same applies to the cords, which may be formed in a large number of configurations. Taking into account these two aspects, it is possible to optimize the design and manufacture of the tires, according to the type of vehicle for which the tire is intended and the associated constraints of use. It is also possible to provide more appropriate arrangements which facilitate certain types of automated manufacture, for example with assembly on a central core and/or without using semi-finished products. It is thus possible to design a tire for a given vehicle which can be manufactured at lower cost.
The anchoring of the reinforcement structure in the bead is ensured, despite the absence of a bead wire of conventional type around which the carcass is usually turned up to create a reliable anchoring link. This type of structure is also advantageous owing to its compactness, and is easy to mount/demount. Furthermore, the traditional carcass upturn which is found in beads of known types, comprising a seat having a generatrix, the axially inner end of which lies on a circle of a diameter greater than the diameter of the circle on which the axially outer end is located, can be dispensed with without sacrificing the integrity, solidity or endurance of the assembly. This aspect contributes to simplifying manufacture, while offering very great latitude in terms of configurations.
Dispensing with the upturn is made possible by using mixes of high modulus in the interface zone in direct contact with the carcass-type reinforcement structure in the anchoring zone. In the conventional tire, the carcasses are in intimate contact with mixes of low or very low modulus, which involves great lengths for transmitting the forces to the bead wire.
According to a particularly advantageous method of manufacture in which the various constituents of the tire are arranged directly on a central core, the shape of which imparts to the tire during manufacture a substantially similar shape to the shape of the finished product, dispensing with the upturn (which exists with a conventional structure) permits advantageous simplification of manufacture.
Preferably, at least one outer flange of a bead is arranged so as to be extended substantially axially and radially towards the outside from the axially outer end of the seat of the bead. For example, the outer flange, which is axially external to the seat, comprises a substantially rectilinear generatrix inclined radially externally relative to the axis of rotation of the tire by an angle of between 30xc2x0 and 85xc2x0, measured from such axis.
Such a flange is normally used together with a rim comprising a rim flange, commonly referred to as a xe2x80x9csidexe2x80x9d, also arranged so as to be extended substantially axially and radially towards the outside from the axially outer end of the seat of the rim. This flange is capable of serving as a bearing zone when the bead is subjected to a force which tends to push it back radially towards the outside. This may be the case, for example, when the phenomenon of rotation of the bead occurs or, alternatively, under the influence of the stress induced during sharp cornering. The flange contributes to holding the bead, and hence the tire, on the rim properly, in particular avoiding any spilling-over axially towards the outside.
According to an advantageous form of embodiment of the invention, the bases of the stacks (the cords radially closest to the axis of rotation of the tire) are arranged radially farther to the outside than the end of the rim flange (axially and radially outermost portion of the flange). The bases of the stacks are advantageously provided so as to be arranged radially externally relative to the flange of the rim which is matched to the tire. The mounting/demounting operations are then facilitated.
Advantageously, the carcass-type reinforcement structure extends substantially radially from each of the beads, along the sidewalls, towards the crown zone. Such structure may thus be unitary and extend from one bead to the other, or alternatively may be divided into two half-structures, each extending along a single sidewall.
According to an advantageous variant, the substantially radial arrangement of circumferential cords is arranged in at least one stack, with each stack being arranged on the axially outer side relative to the carcass-type structure.
According to another advantageous variant, the substantially radial arrangement of circumferential cords is arranged in at least one stack, with each stack being arranged on the axially inner side relative to the carcass-type structure.
According to another advantageous variant, the substantially radial arrangement of circumferential cords is arranged in at least two stacks, the stacks being arranged on either side relative to the carcass-type structure.
The number of stacks and the number of windings or turns of each of the stacks is advantageously established according to the characteristics desired for the tire, for example its operating pressure. For example, a larger number of stacks may be desired in order to increase the rigidity at the level of the zone of the bead.
The carcass-type structure is preferably formed of a cord winding extending back and forth between the two beads, forming loops in each of the beads. Furthermore, the cord winding is preferably formed of a single cord.
According to an advantageous variant, the carcass-type structure forms an extension towards the axis of rotation of the tire beyond the base of the stacks. In such a case, since the upturn of the carcass-type structure is not indispensable for anchoring purposes, the upturn may be made without duplication with circumferential cords running along the structure, and/or without the upturned portion being entirely or partially arranged in a zone of rubber mix of high modulus.
According to another advantageous variant, the inner bead intended to be arranged on the inner side of the wheel and the outer bead intended to be installed on the outer side of the wheel are arranged asymmetrically. Thus, for example, the number of stacks or the number of turns of each of the stacks may be different. Some examples of asymmetrical arrangements are illustrated in the drawings, in which, for example, the number of stacks of cords in the bead on the inner side is different from the number of stacks of cords in the bead on the outer side. For example, the number of stacks of cords in the bead on the inner side is less than the number of stacks of cords in the bead on the outer side. The reverse is also possible, depending on the desired characteristics.
According to another aspect, the symmetry relates to the arrangements of the anchoring, intermediate and bearing zones. Each of the beads may have different architectures in which, for example, the forms, arrangements and dimensions of one or more of the zones may vary. It is also possible to vary the constituent materials, the mechanical properties, such as, for example, hardness, just as the number of zones can be varied.